1. Technical Field
The present invention relates to an atomic oscillator, and particularly to a technique to generate an optimum frequency of EIT to maximize EIT intensity in an atomic oscillator of an EIT system.
2. Related Art
An atomic oscillator based on an electromagnetically induced transparency system (EIT system, also called a CPT system) is an oscillator using a phenomenon (EIT phenomenon) in which when two resonant lights different from each other in wavelength are simultaneously irradiated to an alkali metal atom, the absorption of the two resonant lights is stopped. Only one pair of two resonant lights different in wavelength is prepared, and the frequency is controlled so that the frequency difference (wavelength difference) between the two simultaneously irradiated resonant lights accurately coincides with an energy difference ΔE12 between the respective ground levels. An initial operation at the time when the atomic oscillator in a stop state is started will be described. When the power source of the atomic oscillator is turned on, first, a wavelength of a light source is swept to find the bottom of an absorption band of an objective alkali metal atom. That is, the minimum value of a detection signal is detected when the center frequency of a resonant light pair is swept, that point is determined to be an excitation frequency (excitation wavelength), and an EIT signal is obtained.
U.S. Pat. No. 6,265,945 (patent document 1) discloses a structure and an operation method of a CPT system small atomic oscillator of a sideband system in which a surface emitting laser (VCSEL) is used as a light source.
However, it is experimentally confirmed that the excitation frequency (excitation wavelength) at which the maximum EIT signal intensity is obtained is not the bottom of the absorption band and is shifted to a low frequency side from the bottom of the absorption band (FIG. 4 shows a case where the alkali metal atom is a cesium (Cs) atom, and the EIT signal intensity is maximum at the frequency of point P. The bottom of the absorption band is near 500 (MHz) in the value of the horizontal axis of the drawing. Incidentally, the reference (zero point) of the absolute value of the horizontal axis is arbitrary.). Thus, in the related art atomic oscillator, it can not be necessarily said that the maximum EIT signal is detected, and the degradation of S/N is caused.
Besides, in the related art disclosed in the patent document 1, the frequency condition of two light waves to maximize the CPT (EIT) signal intensity is not prescribed.